The present invention relates generally to stents for implantation in the body to maintain the lumen of a natural (or even artificial) vessel or duct, such as a blood vessel and specifically a coronary artery, open to allow substantially unimpeded flow through the vessel. More particularly, the invention relates to an improved system for delivering a stent to a predetermined site in the vessel for deployment against the inner surface of the vessel wall.
The benefits of angioplasty of arteries, particularly balloon angioplasty, and especially of the coronary arteries, has been amply demonstrated over the past decade. Angioplasty is effective to open occluded vessels that would, if left untreated, result in myocardial infarction (MI) or other cardiac disease or dysfunction. The benefits of the procedure are diminished, however, by restenosis rates approaching 50% of the patient population that undergo the procedure. Accordingly, a huge number of patients experiencing a successful primary percutaneous transluminal coronary angioplasty (PTCA) procedure are destined to require a repeat procedure. The patient faces an impact on his or her tolerance and well-being, as well as the considerable cost associated with repeat angioplasty.
Implantation of coronary stents in angioplasty patients has markedly reduced the complications, risks, potential MI, need for emergency bypass operation, and repeat angioplasty that are prevalent without the stenting procedure. Indeed, to reduce the likelihood of re-obstruction of the vessel at the angioplasty site, it has become common practice for the physician to implant a stent in the patient at the site of the angioplasty or atherectomy procedure, immediately following that procedure, as a prophylactic measure. The stent is advanced on a balloon catheter to the designated site of the prior (or even contemporaneous) procedure, under fluoroscopic observation. When the stent is positioned at the proper site, the balloon is inflated to expand the stent radially to a diameter at or slightly larger (in anticipation of the invariable elastic recoil of the vessel wall following stretching of the wall as a result of implantation of the stent) than the normal unobstructed inner diameter of the arterial wall, for permanent retention at the site. The implant procedure from the time of initial insertion to the time of retracting the balloon is rapid, and much less invasive than coronary bypass surgery.
The typical stent itself is composed of biologically compatible material (biomaterial) such as a suitable metal in wire mesh, wire coil or slotted tubular form. The stent should be of sufficient strength and rigidity to maintain its shape after deployment, and to resist the elastic recoil of the artery after the stent is implanted. Although improved stent structures have been developed, problems persist in the delivery systems by which the stent is implanted at the selected target site.
Successful clinical experience with stent implantation over the past few years has led to much greater use of the device. At some medical institutions, one or more stents are implanted in as many as eighty percent of all interventional procedures for angioplasty in the coronary arteries. While initially only short type AHA-ACC A-type lesions were addressed, more recently highly complex lesions are being treated by interventional strategies. Consequently, patients with multi-vessel disease, as well as those with singular vessel disease, are now candidates for successful revascularization of a narrowed coronary artery system through stenting interventional procedures, without being subjected to a major operation.
A crucial consideration for this type of revascularization is the need for exact placement of the stent at the site of the lesion. Failure to place the stent precisely at the lesion/target site is attributable to either an inability to fully distinguish the location of the stent relative to the lesion on the fluoroscope, or an inability to properly position the stent to the target site despite its viewability. Visibility (i.e., a shadow) of the position of the stent during the implant procedure is impacted by various factors despite the metallic composition of the device. An improved view is attained by providing the stent or the implanting balloon location at which it is mounted with a more distinctive radiopaque marker, or even markers at each end of the stent or at points of demarcation of its position on the balloon. Successful advancement of the stent to the target site, even with a clear view of the stent's location, may nevertheless be adversely impacted by factors such as the length and flexibility of the stent, diameter and profile characteristics of the delivery system including the balloon catheter, surface friction characteristics between the stent and the balloon on which it is mounted, anatomy of the patient, and tortuousness and roughness of the coronary system. Unfortunately, it has not been uncommon for the stent to be dislodged from the balloon or inadequately deployed during the implant procedure, making it necessary to attempt to retrieve or re-position the stent, or to implant a new stent through the improperly deployed one.
It is a principal aim of the present invention to provide a stent delivery system which enables a considerably greater likelihood of successful placement of the stent in a body vessel or duct, even in a tortuous path such as the vascular system leading to or within a coronary artery, than has previously been attained.
A non-expanded stent suitable for implantation in a coronary artery must be quite small, with a diameter less than about one millimeter (mm) and a length (typically a standard length) in a range from about seven to about thirty mm, and having characteristics that allow it to be crimped onto a balloon (either by pre-mounting by machine at the site of manufacture or manual mounting by the physician at the time of the implant procedure) and to be properly expanded in diameter during deployment. Stent structures of the slotted tube type, the coil type, or the wire mesh type are abundantly described in the prior art. Although the traditional method has been manual mounting of the stent on the balloon by the implanting physician at the time of the procedure, by crimping (involving a combination of rolling, squeezing, and compressing the stent), more recently manual mounting has been supplanted by machine mounting of the stent on the balloon at the site of manufacture for immediate use by the physician whenever the implant procedure is to be performed. Pre-mounting eliminates the requirement that the physician be specially skilled in the technique of crimping the stent without damage to the balloon, the stent itself, or capability of symmetrical expansion of the stent.
As noted above, the circumstances and environment for stent placement, particularly at a target site in the coronary vascular system marked by narrow winding arteries, increase the risk that the stent may be dislodged from its mounting balloon. Retrieving a stent which is free to move in the vascular system is a daunting task, requiring all of the skills the physician can muster, and unless accomplished quickly can have calamitous consequences of local blockage, MI and death. Therefore, it is extremely important that the stent be tightly secured to the surface of the balloon to avoid dislodgement, but without deleteriously affecting it from being readily deployed and thereafter released from the balloon at the target site.
The larger the balloon diameter, the easier it is to create friction between the surfaces for better retention of the stent on the balloon. But small diameter balloons, such as those used for PTCA, are more easily advanced through the vascular system. Most stent delivery systems in use today are rather bulky, with typical crossing diameter in the range from 1.4 to 1.6 mm. State of the art PTCA balloons are available with an outer diameter of less than 0.8 mm, which, with stent mounting, equates to a total crossing diameter of less than 1.0 mm.
Accordingly, other important aims of the invention include providing a stent delivery system and method in which the stent is either manually mounted or, preferably, pre-mounted, on the balloon catheter in a way that assures reliable retention of the stent during movement of the delivery system by advancement or withdrawal through the vessel, without interference with deployment of the stent; amenable to use of state of the art PTCA balloons; and with improved fluoroscopic observation of the location of the stent in the delivery system during travel to and placement at the selected target site.